The infectious agent of American trypanosomiasis, Chagas disease, is the haemoflagellate Trypanosoma cruzi. Infection of humans occurs by contamination with freshly infected bug faeces of blood-sucking species of Reduviidae (kissing bugs) or by the bite of an infected bug: - Conjunctivae - Mucous membranes - Skin wounds

T. cruzi undergoes morphogenesis during its life cycle. The trypomastigote and some intermediate forms with a flagellum are found in the peripheral blood. They invade host cells (cardiac and striated muscle fibres) and change to the amastigote form with a truncated flagellum.

The bugs become infected when they feed by biting on a parasitaemic human or animal. In the insect gut, the trypomastigotes change to promastigotes which multiply, change to infectious metacyclic trypomastigotes in the rectum and are then excreted with the faeces.